1. Field of the Invention
The present invention relates to an arc tube and a method for forming an arc tube for a discharge bulb, and more particularly, to an arc tube and method for forming an arc tube that can be used as a light source such as for an automobile headlight.
2. Description of the Related Art
An arc tube is generally used to provide high luminance irradiation. Recently, therefore, an arc tube is frequently used also as a light source for an automobile headlight.
As shown in FIG. 17, an arc tube which is used in an automobile headlight or the like is usually configured by an arc tube body 104 which is made of quartz glass, and in which pinch seal portions 104b are formed in ends of a luminous tube portion 104a constituting a discharge space 102. The arc tube includes a pair of electrode assemblies 106 in which a tungsten electrode 108 and a lead wire 110 are connected and fixed to each other via a molybdenum foil 112. Each of the electrode assemblies 106 is pinch sealed into the arc tube body 104 in the corresponding pinch seal portion 104b with protruding a tip end part of the tungsten electrode 108 into the discharge space 102. As a result of this pinch sealing, the molybdenum foil 112 is joined to the arc tube body 104 in a state where the foil is embedded in the arc tube body 104.
The connection and fixation between the tungsten electrode 108 and the molybdenum foil 112 are performed by welding together the members in a partially overlapped state. In the periphery of the overlapped portion, however, it is not easy to sufficiently ensure a force of joining the molybdenum foil 112 and the arc tube body 104. In a conventional arc tube, therefore, the molybdenum foil 112 is easily peeled off while the arc tube is in use.
When such peeling occurs, the arc tube body 104 cracks, typically starting from an edge of a joining face between the molybdenum foil 112 and the arc tube body 104. As the crack grows, it causes a leakage between the discharge space 102 and the external space. Consequently, the life period of a conventional arc tube becomes relatively short.
The invention has been conducted in view of such circumstances. It is an object of the invention to provide an arc tube and a method for forming an arc tube in which leakage due to peeling of a molybdenum foil is effectively suppressed so that the life period of the arc tube can be prolonged.
The present inventors have discovered that when consideration is given to dimensional relationships among components in the periphery of the overlapped portion of a tungsten electrode and a molybdenum foil, occurrence of peeling of the molybdenum foil may be effectively suppressed without adversely affecting other characteristics. In view of this discovery, an object of the present invention is attained by defining these dimensional relationships.
The arc tube of a first aspect of the invention is an arc tube comprising an arc tube body which is made of, for example, quartz glass, and in which pinch seal portions are formed in ends of a luminous tube portion constituting a discharge space, respectively; and a pair of electrode assemblies in each of which a tungsten electrode and a molybdenum foil are joined together, such as by welding, in a partially overlapped state, and which are pinch sealed into the arc tube body in the pinch seal portions, respectively, with protruding a tip end part of the tungsten electrode into the discharge space, wherein
an overlapped length L1 of the tungsten electrode and the molybdenum foil is set to have the following relationship with a diameter D of the tungsten electrode and a width W of the molybdenum foil:
2Dxe2x89xa6L1xe2x89xa60.8W. 
The arc tube of a second aspect of the invention is an arc tube comprising an arc tube body which is made of quartz glass, and in which pinch seal portions are formed in ends of a luminous tube portion constituting a discharge space, respectively; and a pair of electrode assemblies in each of which a tungsten electrode and a molybdenum foil are welded together in a partially overlapped state, and which are pinch sealed into the arc tube body in the pinch seal portions, respectively, with protruding a tip end part of the tungsten electrode into the discharge space, wherein
a distance L2 between the molybdenum foil and the discharge space is set to have the following relationship with a width A and a thickness B of the pinch seal portion:
Bxe2x89xa6L2xe2x89xa60.8A. 
The above-mentioned xe2x80x9ctungsten electrodexe2x80x9d is an electrode in which tungsten is a principal component.
The above-mentioned xe2x80x9cmolybdenum foilxe2x80x9d is a foil in which molybdenum is a principal component.
Further, the range of xe2x80x9coverlapped length L1xe2x80x9d in the first aspect of the invention, and that of xe2x80x9cdistance L2xe2x80x9d in the second aspect of the invention may be set to both pinch seal portions of the sides of the luminous tube portion, or to only one of the pinch seal portions.
As shown in the above configuration, in the arc tube of the first aspect of the invention, the overlapped length L1 of the tungsten electrode and the molybdenum foil which constitute an electrode assembly of the arc tube is set to be 2Dxe2x89xa6L1xe2x89xa60.8W with respect to the diameter D of the tungsten electrode and the width W of the molybdenum foil. Therefore, the arc tube can attain the following functions and effects.
In each of the electrode assemblies which are pinch sealed into an arc tube body, minute gaps are formed between the electrode assembly and the arc tube body in the periphery of a portion where the tungsten electrode and the molybdenum foil are welded together. The molybdenum foil is peeled off starting from such gaps. As the overlapped length L1 becomes longer, the proportion of the gaps becomes greater, and hence peeling of the molybdenum foil occurs more easily. As a result of temperature changes due to turning-on and turning-off of the arc tube, the tungsten electrode expands and contracts with respect to the arc tube body. As the overlapped length L1 becomes longer, the expansion and contraction of the tungsten electrode produce a greater stress. Also from this viewpoint, peeling of the molybdenum foil occurs easily. Therefore, reduction of the overlapped length L1 to a value which is as small as possible is effective in suppression of occurrence of leakage due to peeling of the molybdenum foil.
By contrast, when the overlapped length L1 is extremely reduced, it becomes physically impossible to join, for example, by welding, the tungsten electrode to the molybdenum foil. In this welding, moreover, there is the possibility that the molybdenum foil is broken in the portion overlapped by the tungsten electrode. Even in the case where the molybdenum foil is not broken in the welding step, when the overlapped length L1 is extremely reduced, the strength of the periphery of the welded portion of the molybdenum foil becomes largely reduced. Therefore, there is the possibility that the molybdenum foil is broken by the pinch seal pressure in the subsequent pinch sealing step, or that, when the arc tube is turned on, the current flow is concentrated in the welded portion to fuse off the welded portion.
When, based on the results of experiments conducted by the present inventors, the overlapped length L1 of the tungsten electrode and the molybdenum foil is set to a value within a range of 2Dxe2x89xa6L1xe2x89xa60.8W with respect to the diameter D of the tungsten electrode and the width W of the molybdenum foil as set forth in the first aspect of the invention, occurrence of peeling of the molybdenum foil can be effectively suppressed while effectively suppressing occurrence of breakage of the molybdenum foil. Therefore, occurrence of leakage due to peeling of the molybdenum foil can be effectively suppressed, so that the life period of the arc tube can be prolonged.
In the arc tube of the second aspect of the invention, the distance L2 between the molybdenum foil which constitutes an electrode assembly of the arc tube, and the discharge space is set to be Bxe2x89xa6L2xe2x89xa60.8A with respect to the width A and the thickness B of the pinch seal portion. Therefore, the arc tube can attain the following functions and effects.
As the distance L2 between the molybdenum foil and the discharge space becomes shorter, the discharge luminous portion (the tip end part of the tungsten electrode) further approaches the molybdenum foil, and hence the temperature change of the overlapped portion of the tungsten electrode and the molybdenum foil becomes larger due to turning-on and turning-off of the arc tube. This increases the expansion and contraction of the tungsten electrode in the overlapped portion with respect to the arc tube body. Therefore, the stress produced in the molybdenum foil is increased so that peeling of the molybdenum foil easily occurs.
By contrast, when the distance L2 between the molybdenum foil and the discharge space becomes long, there arises the following problem. In a region in the vicinity of the joining face with the tungsten electrode in the arc tube body, usually, a crack which radially elongates from the joining face with the tungsten electrode, and that which circumferentially elongates so as to surround the tungsten electrode are caused by the difference in coefficient of thermal expansion between the tungsten electrode and the arc tube body (quartz glass). The former is called electrode cracking. When electrode cracking grows to reach the outer peripheral face of the arc tube body, leakage occurs between the discharge space and the external space. On the other hand, the latter is called bead cracking. When bead cracking is formed, the growth of electrode cracking is blocked. Bead cracking occurs under the condition that the temperature distribution in the axial direction of the area of the pinch seal portion where the tungsten electrode is embedded is approximately even. When the distance L2 between the molybdenum foil and the discharge space is long, however, the temperature distribution in the axial direction of the area of the pinch seal portion where the tungsten electrode is embedded is considerably uneven, so that bead cracking is hardly formed. Therefore, electrode cracking easily grows to reach the outer peripheral face of the arc tube body.
When, based on the results of experiments conducted by the present inventors, the distance L2 between the molybdenum foil and the discharge space is set to be Bxe2x89xa6L2xe2x89xa60.8A with respect to the width A and the thickness B of the pinch seal portion as set forth in the second aspect of the invention, occurrence of peeling of the molybdenum foil can be effectively suppressed while effectively suppressing growth of electrode cracking. Therefore, occurrence of leakage due to peeling of the molybdenum foil can be effectively suppressed, so that the life period of the arc tube can be prolonged.
When an arc tube is configured by a combination of the first and second aspects of the invention, occurrence of peeling of the molybdenum foil can be effectively suppressed while effectively suppressing breakage of the molybdenum foil and growth of electrode cracking. Therefore, occurrence of leakage due to peeling of the molybdenum foil can be effectively suppressed, so that the life period of the arc tube can be further prolonged.